WO2007063342 describes a generally pen-like syringe as shown in FIG. 1 and which is suitable for use with a cartridge containing a medicament. The cartridge is held within a cartridge housing and is adapted to receive a needle (not shown) which pierces a rubber septum sealing a proximal end (with respect to a user's skin) of the cartridge. A cap 1 is provided for covering the cartridge housing 2 and cartridge, and any attached needle. The syringe incorporates a dose metering device provided by a dose knob 3 which is rotatable with respect to the main housing 4 to a position related to the dose of medication to be injected. Rotation of the cap stores energy in a torsion spring 5, which is prevented from unwinding by cooperating splines. The user disengages the splines by means of a sliding trigger 6 coupled to the outside of the main housing 4, resulting in the unwinding of the spring. This in turn causes rotation of a drive gear 7 which is coupled to a plunger 8 via a quick pitch screw thread 9. During firing, the plunger is prevented from rotating by splines in a rewind knob 10 which is rotationally fixed relative to the main housing 4 when the cartridge housing 2 is attached to the main housing 4. The splines interact with recesses along the axial direction of the plunger, such that rotation of the gear results in axial movement of the plunger 8 through the main housing. As illustrated schematically in FIG. 2, a cap 11 mounted on the end of the plunger 8 acts on a “bung” 12 of the cartridge 13, driving medicament 14 from the cartridge, through an attached needle, as the plunger moves in the direction of arrow A.
When a dose has been administered, the cap 11 remains in contact with the bung 12 after driving it forwards. This can lead to leakage of the drug, since the bung is compressed by the action of the cap during the driving stroke, and will tend to expand to its original shape once the stroke is complete. This expansion can force liquid from the cartridge. In addition, the movement of the dose knob 3 when the dose is being set can cause vibrations, and these vibrations may be transmitted down the plunger 8 to the cap 11 and thus the bung 12, again forcing liquid from the capsule.